Silicon carbide (SiC) has a number of characteristics that make it an ideal candidate for a variety of semiconductor applications, primarily those requiring high power handling capabilities. Arguably the most important characteristic of SiC is its indirect bandgap, resulting in relatively high recombination lifetimes and the ability to produce higher voltage junctions than those that can be produced from a direct bandgap material. The large bandgap of this material also provides for negligible current leakage up to 500° C., thereby allowing for high temperature operation without excessive leakage current or thermal runaway. The switching frequency of SiC devices is much higher than that of a device fabricated from silicon or gallium arsenide due to SiC's high breakdown strength and the resultant reduction in minority carrier storage and associated switching losses. Lastly, due to the high junction temperature and the high thermal conductivity of SiC, devices fabricated from SiC have reduced cooling requirements.
Although semiconductor devices based on SiC offer vast improvements over devices fabricated from silicon, in order to realize these improvements materials must be fabricated with much lower defect densities than have been obtainable heretofore. As noted by the authors in the 1999 article entitled SiC Power Devices, Naval Research Reviews, Vol. 51, No. 1 (1999), in order to scale up devices fabricated from SiC, the density of dislocations as well as the density of micropipes must be reduced. Conventional SiC material has a dislocation density between 105 and 106 per square centimeter and a micropipe density between 102 and 103 per square centimeter. Some extremely high quality SiC material has been grown with dislocation densities on the order of 104 per square centimeter. Unfortunately, even this dislocation density is at least an order of magnitude too high for many semiconductor applications. Id. at page 21.
U.S. Pat. No. 5,679,153 discloses a technique of growing SiC epitaxial layers using liquid phase epitaxy in which the density of micropipes is substantially reduced or eliminated. In one aspect of the disclosed technique, an epitaxial layer of SiC is formed on a bulk single crystal of SiC, the epitaxial layer being of sufficient thickness to close micropipe defects propagated from the bulk crystal. In order to form an electronically active region for device formation, a second epitaxial layer is formed on the first epitaxial layer by chemical vapor deposition. Based on this technique, SiC layers having micropipe densities of between 0 and 50 micropipes per square centimeter on the surface were claimed.
Although techniques have been disclosed to achieve SiC materials with low micropipe densities, these techniques do not lend themselves to growing bulk materials, i.e., materials that are at least a millimeter thick or more preferably, at least a centimeter thick. Additionally, these techniques do not impact the dislocation densities of the material. Accordingly, what is needed in the art is a technique of growing bulk SiC material with defect densities on the order of 103 per square centimeter, more preferably on the order of 102 per square centimeter, and even more preferably on the order of 10 or less dislocations per square centimeter. The present invention provides such a technique and the resultant material.